Our team has experienced deleted responses coming back to life, so what we typically do now is disqualify the responses we do not want to use first, and then as step 2 we go on to delete them. To do this:

Start by navigating to the “Results” tab and select “Individual Responses”:

This way even if responses become un-deleted by error, they will no longer count toward your quota because they are now accounted for as “Disqualified” rather than “Complete Responses”. I hope this helps!